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United States Patent |
5,668,516
|
Xu
,   et al.
|
September 16, 1997
|
Simplified active shield superconducting magnet assembly for magnetic
resonance imaging
Abstract
An improved actively shielded superconducting magnet in which the main and
bucking coils are directly wound onto respective coil support structures
made of glass fiber-reinforced epoxy. The main and bucking coil cartridges
are held in a fixed concentric relationship via a pair of flanges located
at opposite ends of a helium vessel. During manufacture of the main and
bucking coil cartridges, the outer diameters of the respective coil
support structures are machined with high precision. Also, the diameters
of two concentric grooves are precisely machined on the inner surface of
each helium vessel end flange to match the outer diameters of the main and
bucking coil support structures, so that at room temperature the main and
bucking coil cartridges can slide smoothly into these grooves. The helium
vessel is made of aluminum alloy and has a coefficient of thermal
expansion which is greater than that of the fiber-reinforced epoxy coil
support structures. When the helium vessel is filled with liquid helium,
the helium vessel end flanges hold the main and bucking coil cartridges
tightly with predetermined amount of interference due to differential
contraction of the aluminum alloy and the fiber-reinforced epoxy
materials. This provides mechanical support to the main and bucking coil
cartridges and ensures their concentricity. The relative axial positions
of the main and bucking coil cartridges are fixed using the same working
principle.
Inventors:
|
Xu; Bu-Xin (Florence, SC);
Lochner; Ronald F. (Florence, SC)
|
Assignee:
|
General Electric Company (Milwaukee, WI)
|
Appl. No.:
|
581100 |
Filed:
|
December 29, 1995 |
Current U.S. Class: |
335/216; 62/51.1; 324/318; 335/299; 335/301 |
Intern'l Class: |
H01H 001/00 |
Field of Search: |
335/216,296,299,301
324/318,319,320
62/51.1
|
References Cited
U.S. Patent Documents
4771256 | Sep., 1988 | Laskaris et al. | 335/301.
|
5099215 | Mar., 1992 | Woods et al. | 335/216.
|
5237300 | Aug., 1993 | Ige et al. | 335/299.
|
Primary Examiner: Donovan; Lincoln
Attorney, Agent or Firm: Flaherty; Dennis M., Pilarski; John H.
Claims
We claim:
1. A magnet assembly comprising:
a vessel comprising opposed first and second end flanges each having
mutually concentric first and second annular grooves formed on an inner
face thereof, an outer cylindrical wall having first and second ends
respectively connected to said first and second end flanges, and an inner
cylindrical wall having first and second ends respectively connected to
said first and second end flanges, said inner cylindrical wall being
surrounded by said outer cylindrical wall, and said first annular groove
having a radius less than said second annular groove;
a circular cylindrical outer coil support contained in said vessel and
having first and second ends inserted in said second annular grooves of
said first and second end flanges respectively, said outer coil support
having a first circumferential groove formed on its outer surface;
a circular cylindrical inner coil support contained in said vessel and
having first and second ends inserted in said first annular grooves of
said first and second end flanges respectively, said inner coil support
having a first circumferential groove formed on its outer surface;
a first main magnet coil arranged in said first circumferential groove in
said inner coil support; and
a first bucking magnet coil arranged in said first circumferential groove
in said outer coil support.
2. The magnet assembly as defined in claim 1, wherein said inner and outer
coil supports are made of glass fiber-reinforced epoxy.
3. The magnet assembly as defined in claim 2, wherein said inner and outer
cylindrical walls and said first and second end flanges are made of
aluminum alloy.
4. The magnet assembly as defined in claim 1, wherein said first main
magnet coil is wound directly on said inner coil support and said first
bucking magnet coil is wound directly on said outer coil support.
5. The magnet assembly as defined in claim 1, wherein said outer coil
support has a second circumferential groove formed on its outer surface
and said inner coil support has a second circumferential groove formed on
its outer surface, further comprising a second main magnet coil arranged
in said second circumferential groove in said inner coil support, and a
second bucking magnet coil arranged in said second circumferential groove
in said outer coil support.
6. The magnet assembly as defined in claim 1, wherein each of said first
and second end flanges is an annular plate having a radially outer
periphery and a radially inner periphery.
7. The magnet assembly as defined in claim 6, wherein said first and second
ends of said outer cylindrical wall are welded to said radially outer
peripheries of said first and second end flanges respectively, and said
first and second ends of said inner cylindrical wall are welded to said
radially inner peripheries of said first and second end flanges
respectively.
8. The magnet assembly as defined in claim 1, further comprising a latch
attached to said first end flange by a threaded fastener, said latch
having a projection which abuts a portion of said inner coil support and
blocks axial displacement of said portion of said inner coil support in a
direction away from said first end flange.
9. The magnet assembly as defined in claim 1, further comprising a key
welded to said first end flange, said key having a projection which fits
in a recess formed in a portion of said outer coil support and blocks
axial displacement of said portion of said outer coil support in a
direction away from said first end flange.
10. The magnet assembly as defined in claim 1, further comprising a key
attached to said first end flange by first and second threaded fasteners,
said key being configured and arranged such that a first end of said key
projects into a groove in said inner coil support and a second end of said
key projects into a groove in said outer coil support.
11. A magnet assembly comprising:
a vessel comprising mutually concentric inner and outer circular
cylindrical walls, an annular volume between said inner and outer circular
cylindrical walls being closed by first and second annular plates
connected to said inner and outer circular cylindrical walls;
a circular cylindrical outer coil support contained in said vessel and
having first and second end faces which contact said first and second
annular plates respectively, said outer coil support having a
circumferential groove formed on its outer surface;
a circular cylindrical inner coil support contained in said vessel and
having first and second end faces which contact said first and second
annular plates respectively, said inner coil support having a
circumferential groove formed on its outer surface;
a first magnet coil arranged in said circumferential groove in said inner
coil support; and
a second magnet coil arranged in said circumferential groove in said outer
coil support,
wherein said inner and outer circular cylindrical walls are made of a first
material having a first coefficient of thermal expansion, and said inner
and outer coil supports are made of a second material having a second
coefficient of thermal expansion, said second coefficient of thermal
expansion being less than said first coefficient of thermal expansion.
12. The magnet assembly as defined in claim 11, wherein said first material
is metal alloy and said second material is reinforced epoxy.
13. The magnet assembly as defined in claim 12, wherein said metal alloy is
aluminum alloy and said reinforced epoxy is reinforced with glass fiber.
14. The magnet assembly as defined in claim 11, wherein said first magnet
coil is wound directly on said inner coil support and said second magnet
coil is wound directly on said outer coil support.
15. The magnet assembly as defined in claim 11, wherein each of said first
and second annular plates has mutually concentric first and second annular
grooves formed on one side thereof, said inner coil support has first and
second ends inserted in said first annular grooves of said first and
second end flanges respectively, and said outer coil support has first and
second ends inserted in said second annular grooves of said first and
second end flanges respectively.
Description
FIELD OF THE INVENTION
This invention relates to support structures for superconducting magnets.
In particular, the invention relates to structures for supporting magnet
coils of an active magnetic shielding system.
BACKGROUND OF THE INVENTION
As is well known, a coiled magnet, if wound with wire possessing certain
characteristics, can be made superconducting by placing it in an extremely
cold environment, such as by enclosing it in a cryostat or pressure vessel
containing liquid helium or other cryogen. The extreme cold reduces the
resistance in the magnet coils to negligible levels, such that when a
power source is initially connected to the coil (for a period, for
example, of 10 minutes) to introduce a current flow through the coils, the
current will continue to flow through the coils due to the negligible
resistance even after power is removed, thereby maintaining a magnetic
field. Superconducting magnets find wide application, for example, in the
field of magnetic resonance imaging (hereinafter "MRI").
The modern MRI system requires a high-strength uniform magnetic field in a
large imaging volume, for example, a field having an inhomogeneity of a
few parts per million over a spherical volume having a diameter of 40-50
cm. The signal-to-noise ratio of MRI is proportional to the field strength
in the imaging region. To have a high-quality image, the field strength
for MRI is usually required to be larger than 0.5 T and up to 2 or 3 T. On
the other hand, the stray field produced by such a magnet must be limited
to a small volume to minimize the environmental impact of a magnet of such
large size. For example, since an MRI system is often installed in
hospitals, which contain various electronic equipment and extraneous
magnetic fields surrounding the MRI system location, the equipment must be
isolated from the MRI magnetic field and the MRI system must be shielded
from surrounding magnetic fields. Generally, the 5-Gauss line of a MRI
magnet cannot be over about 2.5 m radially and 4.0 m axially. Active
shield superconducting magnet technology was developed to meet the
foregoing design goals.
A typical active shield magnet consists of two sets of superconducting
coils. An inner set of coils, usually called the main magnet coils,
produce a uniform magnetic field of large magnitude in a imaging volume.
The conventional support structure for the main magnet coils is a circular
cylindrical aluminum drum. The main coils are wound separately around
stainless steel bobbins, placed in grooves machined in the drum and spaced
axially along the inside of the drum. Another set of outer magnet coils,
usually called bucking coils, are spaced from and surround the main coils,
and are supported by a structure which is secured to the drum. The bucking
coils carry currents in the direction opposite to the direction of
currents being carried by the main coils so as to cancel the stray
magnetic field outside the magnet. This is called active magnetic
shielding.
However, in the process of energizing the magnets, or ramping the magnets
to field, and in cooling the coils to superconducting temperatures, the
coils are subjected to significant thermal and electromagnetic loading. As
a result, actively shielded magnets pose difficult problems in terms of
structural support. The principal reason for utilizing actively shielded
magnets, as opposed to a passively shielded system, is that the latter
would require massive amounts of magnetic material, such as iron, around
the magnet, which would increase both the weight and volume of the system
considerably. To minimize weight and volume through use of bucking coils,
and thus realize the objectives of active magnetic shielding, it is
important that the support structure for the bucking coils be relatively
lightweight and yet withstand the significant magnetic and thermal loads
placed upon it during energization and operation of an MRI system. It is
also important that the bucking coils maintain close positional accuracy,
notwithstanding the significant thermal loads during initial operation or
cooldown of the MRI, and notwithstanding the electromagnetic loads
generated during energization and operation. As a result, there are
conflicting thermal, magnetic and mechanical considerations and factors
which must be balanced and compromised to obtain an acceptable bucking
coil assembly.
Thus, a conventional active shield magnet has a very complicated structure
and has been expensive to build. An example of a known active shield
magnet is described in U.S. Pat. No. 5,237,300. In accordance with that
arrangement as shown in FIG. 1 annexed hereto, a plurality of main magnet
coils 4a-4f produce a highly uniform magnetic field of large magnitude.
The main coils are supported by a circular cylindrical drum 2 within
machined pockets or grooves. Drum 2 is typically made of aluminum alloy. A
pair of bucking coils 10a and 10b concentrically surround portions of drum
2 and main magnet coils 4a-4f. The bucking coils carry currents in a
direction opposite to the direction in which currents are carried by the
main magnet coils, thereby providing cancellation of magnetic fields in
the region outside the MRI system. The bucking coils are supported on
bucking coil support cylinders or bands 6 and 8, respectively. The
supports for the bucking coils include a plurality of struts or plates 12
extending angularly outward from drum 2, or from bands 16 around the drum,
to bucking coil support cylinders 6 and 8, with spacing rods 14 extending
axially between the bucking coil support cylinders.
SUMMARY OF THE INVENTION
The present invention is an improved actively shielded superconducting
magnet in which the main coils are directly wound onto the outer
circumferential surface of a circular cylindrical coil support structure
made of glass fiber-reinforced epoxy. Similarly, the bucking coils are
directly wound on the outer circumferential surface of another circular
cylindrical coil support structure made of glass fiber-reinforced epoxy
and having a diameter greater than the diameter of the main coil support
structure. Several layers of aluminum overwrap tape are wrapped on top of
the main and bucking coil windings to constrain radially outward
displacement of the coils during magnet energization.
In accordance with a preferred embodiment of the invention, the main coil
cartridge and the bucking coil cartridge are held in a fixed concentric
relationship via a pair of flanges located at opposite ends of a helium
vessel. During manufacture of the main and bucking coil cartridges, the
outer diameters of the respective coil support structures are machined
with high precision. Also, two concentric grooves are machined on the
inner surface of each helium vessel end flange. The respective diameters
of these two grooves are precisely machined to match the outer diameter of
the main and bucking coil support structures, so that at room temperature
the main and bucking coil cartridges can slide smoothly into these
grooves.
The helium vessel is made of aluminum alloy and has a coefficient of
thermal expansion which is greater than that of the fiber-reinforced epoxy
coil support structures. When the helium vessel is filled with liquid
helium, the helium vessel end flanges hold the main and bucking coil
cartridges tightly with a predetermined amount of interference due to
differential contraction of the aluminum alloy and the fiber-reinforced
epoxy materials. This not only provides mechanical support to the main and
bucking coil cartridges, but also completely fixes the relative radial
positions of the main and bucking coil cartridges so that their
concentricity is assured.
The relative axial positions of the main and bucking coil cartridges are
fixed using the same working principle. The depth of the grooves on the
helium vessel end flanges is also precisely machined. During magnet
assembly, one of the helium vessel end flanges is first positioned in a
flat platform. The main and bucking coil cartridges are each vertically
inserted and seated on the flange grooves and then locked by a set of
keys. The inner and outer cylinders and the other end flange of the helium
vessel are then assembled and welded together. After cooldown of helium
vessel and magnet assembly, the axial differential contraction between the
aluminum alloy helium vessel and the fiber-reinforced epoxy coil support
structures will close any axial gaps between the helium vessel end flanges
and the coil support structures and provide a firm support to the main and
bucking coil cartridges.
The helium vessel/magnet assembly is then installed in a typical cryogenic
vacuum enclosure which is fitted with peripheral equipment to produce
magnetic resonance images.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram depicting a side sectional view of a
conventional actively shielded superconducting magnet assembly.
FIG. 2 is a schematic diagram depicting an interior end view of a helium
vessel/magnet assembly in accordance with the preferred embodiment of the
invention.
FIGS. 3-5 are schematic diagrams depicting respective partial sectional
views of the helium vessel/magnet assembly shown in FIG. 2, with the
sections being taken along lines 3--3, 4--4 and 5--5, respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 2, the magnet assembly in accordance with the preferred
embodiment of the invention comprises a leaktight vessel filled with
liquid helium or other cryogen (hereinafter "helium vessel"). The helium
vessel comprises a pair of mutually parallel, opposed end flanges 24, only
one of which is shown in the drawings. Each end flange is an annular
plate, preferably fabricated from aluminum alloy. The helium vessel
further comprises an outer cylindrical wall 20 and an inner cylindrical
wall 22, also made of aluminum alloy. Walls 20 and 22 are mutually
concentric, with wall 20 having a radius greater than that of wall 22. As
seen in FIG. 3, the ends of wall 20 are respectively welded to the outer
peripheries of the end flanges 24 (weld W1), while the ends of wall 22 are
respectively welded to the inner peripheries of the end flanges (weld W2).
The final enclosed welded assembly has a generally toroidal shape.
In accordance with the preferred embodiment of the invention, the helium
vessel contains a main coil cartridge 26 and a bucking coil cartridge 28
(see FIG. 2). Both cartridges are generally circular cylindrical, with the
radius of cartridge 28 being greater than the radius of the cartridge 26.
The main coil cartridge 26 has an inner circumferential surface of radius
greater than the radius of the outer circumferential surface of the helium
vessel inner cylindrical wall 22. Similarly, the bucking coil cartridge 28
has an outer circumferential surface of radius less than the radius of the
inner circumferential surface of the helium vessel outer cylindrical wall
20.
As shown in FIG. 3, the main coil cartridge 26 comprises a main magnet coil
support 27 and a plurality of main magnet coils (typically six or more)
arranged in annular grooves formed on the outer circumferential surface of
main magnet coil support 27. FIG. 3 shows only three of those main magnet
coils designated 36a, 36b and 36c. The main magnet coils are directly
wound on coil support 27 with specified tensions (typically 30-60 lbs.).
Similarly, the bucking coil cartridge 28 comprises a bucking magnet coil
support 29 and a plurality of bucking magnet coils (typically two or more)
arranged in annular grooves formed on the outer circumferential surface of
bucking magnet coil support 29. FIG. 3 shows only one of those bucking
magnet coils designated 38. The bucking magnet coils are directly wound on
coil support 29 with the same specified tensions. The coil support
structures 27 and 29 are both made of glass fiber-reinforced epoxy.
Several layers of aluminum overwrap tape 40 (typically 4 to 8 layers) are
wrapped on top of the main and bucking coil windings to constrain radially
outward displacement of the coils during magnet energization.
The coil supports 27 and 29 are held in a fixed concentric relationship by
the end flanges 24 located at opposite ends of the helium vessel. During
manufacture of the main and bucking coil cartridges, the outer diameters
of the respective coil supports 27 and 29 are machined with high
precision. Also, two concentric grooves are machined on the inner surface
of each helium vessel end flange 24. The respective diameters of these two
grooves are precisely machined to match the outer diameter of the main and
bucking coil supports 27 and 29, so that at room temperature the main and
bucking coil cartridges can slide smoothly into these grooves. The ends of
main magnet coil support 27 are inserted in the smaller-radius annular
grooves formed in the respective end flanges 24, and the ends of bucking
magnet coil support 29 are inserted in the larger-radius annular grooves
formed in the respective end flanges, as shown for one end flange in FIG.
3.
The helium vessel is made of aluminum alloy and has a coefficient of
thermal expansion which is greater than that of the fiber-reinforced epoxy
coil support structures. When the helium vessel is filled with liquid
helium, the temperature of the helium vessel and magnet cartridges therein
drops, causing the respective materials to contract. Due to differential
contraction of the aluminum alloy and the glass fiber-reinforced epoxy,
the helium vessel contracts more than the coil supports. One result of
this differential thermal contraction is that each helium vessel end
flange 24 holds the main and bucking coil cartridges 26 and 28 tightly
with a predetermined amount of interference. This provides mechanical
support to the main and bucking coil cartridges, and also fixes the
relative radial positions of the main and bucking coil cartridges so that
their concentricity is assured.
Another result of differential thermal contraction is that the relative
axial positions of the main and bucking coil cartridges are fixed. During
fabrication of the helium vessel end flanges, the depth of the annular
grooves on each end flange 24 is precisely machined. During magnet
assembly, one of the helium vessel end flanges is placed flat on a
platform. Then the main and bucking coil cartridges are each vertically
inserted in a respective annular groove until the radial endfaces of the
inserted ends of the coil supports are seated on the bottom walls of the
annular grooves in the end flange. The main magnet coil cartridge 26 is
then locked against the end flange 24 by installing a plurality of latches
30 around the circumference of the main magnet coil cartridge 26, as seen
in FIG. 2.
Referring to FIG. 5, each latch 30 is attached to the end flange 24 by a
threaded fastener 44. The latch 30 has a projection which abuts lip 27a
when fastener 44 is fully tightened, so that at that location the
projection blocks axial displacement of inner coil support 27 in a
direction away from the end flange. Installation of a plurality of latches
30 at intervals around the circumference (see FIG. 2) serves to lock the
main magnet coil cartridge 26 against axial displacement relative to the
end flange 24, as well as preventing yawing of cartridge 26 relative to
the vessel centerline during subsequent assembly procedures.
The bucking magnet coil cartridge 28 is held axially against the end flange
24 by means of a T-shaped key 34, shown in side profile in FIG. 4. The
side of the key head confronting the end flange has a chamfer which
defines a weld groove filled with weld material to form weld W3, by which
key 34 and end flange 24 are joined. The shaft of key 24 fits snugly in a
radial bore 46 of circular cross section which is machined in the bucking
magnet coil support 28. Thus, when key 34 is welded to the end flange 24,
axial displacement of outer coil support 29 in a direction away from the
end flange is blocked at that location. Installation of a plurality of
latches 34 at intervals around the circumference (see FIG. 2) serves to
lock the bucking magnet coil cartridge 28 against axial displacement
relative to the end flange 24, as well as preventing yawing of cartridge
28 relative to the vessel centerline during subsequent assembly
procedures.
Referring to FIG. 2, relative circumferential displacement of magnet coil
cartridges 26 and 28 is prevented by installation of a pair of bar-shaped
keys 32. Each key 32 is attached to the end flange 24 by means of a pair
of threaded fasteners 42 (see FIG. 2). Key 32 is provided with a rounded
radial slot 52, through which the shafts of fasteners 42 pass. In the
installed position, one end of key 32 is interlocked with an axial groove
48 formed in lip 27a of inner coil support 27, and the other end of key 32
is interlocked with a radial groove 50 formed in the endface of outer coil
support 29 which contacts the radially outer annular groove in the end
flange.
After the cartridges 26 and 28 are secured to the end flange 24, the inner
cylindrical wall 22, the outer cylindrical wall 20 and the other end
flange (not shown) of the helium vessel are assembled and welded together.
After cooldown of the helium vessel and magnet assembly, the axial
differential contraction between the aluminum alloy helium vessel and the
fiber-reinforced epoxy coil supports will close any axial gaps between the
helium vessel end flanges and the coil supports and provide a firm support
to the main and bucking coil cartridges.
The preferred embodiment of the invention has been disclosed for the
purpose of illustration. Variations and modifications which do not depart
from the broad concept of the invention will be readily apparent to those
skilled in the design of actively shielded superconducting magnets. All
such variations and modifications are intended to be encompassed by the
claims set forth hereinafter.
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